1. Field of the Invention
The present invention relates to novel organometallic polymers based on boron and nitrogen and to a process for the synthesis of such novel polymers.
This invention also relates to the use of such novel organometallic polymers in the production of ceramic materials and shaped articles based on boron nitride, especially boron nitride in fibrous form.
The present invention lastly relates to novel intermediate compositions which are useful starting materials in the synthesis of the aforenoted novel organometallic polymers.
2. Description of the Prior Art
Boron nitride is increasingly in demand in this art in light of its high thermal stability, its impact strength, its great chemical inertness and its very good thermal conductivity. Furthermore, its low electrical conductivity makes it an insulating material of choice.
Several processes are presently known to the art for the preparation of boron nitride.
One such process includes reacting boron trichloride with ammonia in the gaseous state. A fine boron nitride powder is obtained in this manner, which may be sintered to produce solid shaped articles. However, the shaped articles thus produced exhibit a characteristic microporosity which may be highly detrimental for certain applications.
More recently, it was discovered that boron nitride could be produced by the pyrolysis of precursor polymers.
The advantage of this "polymer" method primarily resides in the form of the final product, and, more particularly, enables the production, after pyrolysis, of boron nitride fibers.
Thus, U.S. Pat. No. 4,581,468 describes a preceramic organoboron polymer prepared by the interaction of ammonia (ammonolysis) with a B-trichloro-N-tris(trialkylsilyl)borazine (a cyclic compound) and which, as indicated, after drawing and pyrolysis at 970.degree. C., results in the production of boron nitride fibers.
However, the beginning cyclic compound described in the patent is difficult to prepare and is thus expensive. Further, the maximum yield of such compound does not exceed 22% by weight, indicating actual yields appreciably lower than 22%. Consequently, it is not realistically suitable for applications on an industrial scale.